Generally, cranial stabilization refers to a line of compatible and interconnectable medical devices used during neurosurgery to hold the head of a patient in a fixed position relative to a surgical operating table. A typical arrangement of such products may include, for example, a base unit that connects directly to the surgical table, one or more adaptors connected to the base unit, and a skull clamp connected to an adaptor. The skull clamp often has three skull pins that engage and hold the skull of the patient.
One type of known skull clamp is designed to accommodate all sizes of heads and is comprised of two opposed clamp arms that have proximal ends slidable with respect to each other. A distal end of one clamp arm has a single skull pin, and a distal end of the other clamp arm has a rocker arm with two skull pins. Such a skull clamp and its supporting linear slide mechanism is a relatively large structure. Scanning machines require that a skull clamp and supporting structure be fully radiolucent, as small as possible and still provide flexibility and stability in supporting a patient's skull. One known approach to reducing the size of the skull clamp is to replace the two opposed sliding clamp arms with a radiolucent unitary clamp body structure.
Regardless of the general construction of the skull clamp, three skull pins are used to support the head of a patient. One skull pin is positioned to contact one side of the head; and two skull pins are positioned to contact an opposite side of the head. It is often desirable that the clamping force applied by the skull pins be distributed so that equal and opposite forces are applied to opposing sides of the head. With known designs, a force adjusting screw is threaded through the skull clamp body. A skull pin is inserted with a compression spring in an inner end of the force adjusting screw. Thus, with the skull pin contacting the head, as the force adjusting screw is tightened, the compression spring is compressed; and the skull pin applies an engagement force against the head. The magnitude of the engagement force is determined by the spring constant of the spring. A scale often extends through an outer end of the adjusting screw; and as the adjusting screw is tightened and the engagement force increases, the scale extends past the outer end of the adjusting screw an amount proportional to the engagement force. Thus, by observing the scale, the adjusting screw can be tightened to provide a desired engagement force magnitude.
While the above skull pin and force adjusting screw combination works well, it does have disadvantages. First, the compression spring within the force adjusting screw is made of metal and thus, is not radiolucent. The use of nonradiolucent materials in association with a radiolucent skull clamp body is undesirable and results in artifacts in MRI scans of the head when it is mounted in the skull clamp. Further, the manifestation of such artifacts is unpredictable and may vary from scan to scan, which makes the scanned information more difficult to interpret and use. In addition, the artifacts increase as the number of skull pins with an engagement force indicator increases. In addition, the force adjusting screw and scale extend radially outward beyond the outer perimeter of the skull clamp, which increases the envelope of space required by the skull clamp within the scanning machine.
Thus, there is a need to provide a skull pin force adjusting mechanism that does not have the disadvantages discussed above.